The present invention concerns an electrolytic or galvanic sulfite containing bath, free from cyanide, for the electrodeposition of palladium and alloys thereof. The invention also concerns a process for the preparation and the replenishing of said electrolytic bath.
There exists already many electrolytic baths and methods for coating conductive pieces, or pieces made electrically conductive, with an electrolytic deposit of palladium or of alloys of palladium with precious or base metals. For the preparation of such baths, a great variety of salts have been employed and, more particularly, complexes the degree of coordination of which is often ill defined.
Thus, baths prepared from complex palladium cyanides have been used, e.g., Pd(NH.sub.3).sub.2 (CN).sub.2 (U.S. Pat. No. 1,991,995), from palladium nitrates in ammonia (Electroplat. Met. Finishing (1962) 15, 20) and from palladium and potassium nitrate (U.S. Pat. No. 1,993,623).
Halogenated derivatives have also been used for such baths, e.g., palladium tetrammine halides (German Pat. No. 1,262,722, Russian Pat. No. 280,153 and British Pat. No. 1,143,178) and palladium dichlorodiamine hydroxide (French Pat. No. 1,417,567).
Palladium nitrite baths are also known (Metal Finishing Guidebook & Directory, Westwood, N.J., USA, pp. 335-337), as well as those containing palladium tetrammine hydroxide (Russian Pat. No. 291,988 or complexes of palladium with organic compounds, e.g., palladium cyclohexanediamine tetraacetate, palladium ethylenediamine chloride and the corresponding sulfate (British Pat. No. 1,051,383) and salts of palladium and urea (German patent application No. 1,796,110).
The baths described above generally have a pH neutral or basic, acidic baths being apparently more difficult to control. However, a strongly acidic bath containing palladium nitrate and a small proportion (5 - 20%) of palladium sulfite has been recently disclosed (German patent application No. 2,105,626).
Although the baths described above have some advantages, they also have drawbacks. Thus, in general, palladium and palladium alloy deposits obtained from the known methods are subject to high internal stress which can result in the formation of splits and cracks when the thickness of the deposit is more than a certain critical value, e.g., 5 to 10.mu.. Such deposits are also very porous.
Furthermore, the use of palladium nitrate or nitrite baths, particularly in the presence of sulfites and organic chelating agents or brighteners, sometimes results in some undesirable discoloring of the deposits.
Also, in halogen rich baths, free halogens are released at the anodes during electrolysis which oxidizes said anodes and shorten the useful life thereof.
Finally, it should be noted that the presence of cyanides in electrolytic baths is highly undesirable because of their inherent toxicity and the problems associated with the treatment of waste liquors.